1. Field of Invention
The invention relates to the foundry industry where common practice today is to purchase new clean sand found in natural deposits to replace a portion of sand used in the production of molds and cores where organic and inorganic contaminents from binders and other additives have built up to an objectionable level. As an alternative, "thermally reclaimed" sand (i.e. sand that is heated to a high enough temperature to partially or completely remove organic material, and alter properties of inorganics so that they may be removed more easily by some other means, such as mechanical scrubbing) may be used in lieu of purchasing new sand and dumping contaminated sand. In this particular case elevated temperature is required to promote certain chemical and physical reactions, but not necessary in the end product. In most cases elevated temperature would actually be detrimental, because typical foundry processes require sand to be at or near ambient temperature for the production of molds and cores. Although the invention is based upon conditions and processes described above, it may find application in other areas, such as calcining, or ore roasting.
2. Description of Prior Art
In searching for prior art, U.S. Pat. No. 2,788,204 discloses a device that is designed, to accomplish essentially the same result, although by substantially different means. Similarities include using a rotating drum and having an outside heat source. However, heat transfer between incoming and exiting material is being accomplished through a series of chambers that have walls perpendicular to the axis of rotation with small openings or passageways for the material to move from one compartment to another. This device has several inherent disadvantages: 1. Construction of the heat exchange chamber would be difficult and costly. 2. Once assembled, maintenance on inner compartments would almost by impossible.
U.S. Pat. No. 822,022 discloses a double wall rotating drum device that is essentially an oven designed to dry sand and gravel. Similarities between this device and the present invention include using a double wall rotating drum and an outside heat source. However the device, as designed, will not accomplish effectively the results desired. First, the heat source is located at the same end at which feed material enters and exits. This will tend to promote the opposite effect wanted. In this situation the incoming material and the exiting material are closest to the heat source. The point where material transfers from one chamber to another is farthest away from the heat source. The net effect would be a rather "flat" temperature curve if one would monitor the temperature of a particle of material going through the device and plot temperature vs. time. If there is heat transfer between the two chambers, it would even be questionable which way the transfer is taking place. Close inspection at the feed end might show that while the incoming material is colder than the outgoing material, the actual temperature of the inner wall at this point, being in direct contact with combustion gases, may be higher than either material, thereby causing a transfer of heat from the inner wall to the out going material, which would result in the opposite effect wanted.
Besides the location of the heat source, other design characteristics of this device do not lend themselves to accomplish the results desired. This device is designed to heat relatively large tonnages of material to a relatively low temperature for the removal of moisture. A relatively large portion of the heat produced by the heat source is used for heat of evaporation. The present invention is required to heat the material to a much higher temperature. In the case of thermally reclaiming foundry sands, a maximum temperature of 1500 F. or even higher would not be uncommon. Since the thermal conductivity of sand is very poor, maximum temperatures required are high, and in the end an elevated temperature of the exit material is not required; certain other design characteristics must be maintained for which U.S. Pat. No. 822,022 does not take into consideration.
The following factors must be controlled and optimized in order efficiently recuperate heat between outgoing and incoming materials:
1. Maintaining a rather slow flow rate of feed material through the inner and outer chambers. PA1 2. Maintaining a large volume of material in both chambers (which in combination with the slow flow rate will provide a high retention time.) PA1 3. Keeping the feed material in as much motion as possible, done in such a manner as to also keep material homogenized thereby minimizing temperature gradients through the material. PA1 4. Providing the best conditions possible for maximum heat transfer utilizing concepts of convection, radiation, and direct conduction. PA1 5. Providing a condition where the inner wall can be made very thin, yet structurally sound enough to support the load, thereby maximizing heat transfer through the wall. PA1 6. Providing a means of isolating as much as possible the high temperature heat source from directly heating the incoming material at the feed inlet end, while still recuperating as much heat as possible from the flue gas (if heated with a combustion burner.) PA1 7. If in a particular case where the feed material may contain organics, and if in the process of heating, usable gases are given off, an efficient means of combusting these gases and utilizing the heat given off from the same.
It is to these ends unique solutions have been developed and designed, and consequentially embody the present invention.